A study will be made of the regulation of energy conversion in the heart by the peptide inhibitor of mitochondrial ATPase. The inhibitor is found in the mitochondrion in close association with the ATPase enzyme where, it has been suggested, it may play an important role in the control of the synthesis and utilization of ATP. This project will focus on two key aspects of the problem. The first is the role of the inhibitor in oxidative phosphorylation. Measurements will be made of the amount of the inhibitor in mitochondria and submitochondrial particles, of putative energy-dependent effects on the interactions of the ATPase and the peptide inhibitor and of a possible requirement for the inhibitor in ATP-dependent reactions of oxidative phosphorylation. An important element of the program will be the use of an antibody, raised against the inhibitor, to monitor as well as to manipulate the systems to be studied. The second aspect of the projct will be a study of the physical and chemical properties of the inhibitor which is intended to elucidate the modes of interaction of the peptide with the ATPase.